Field effect transistor with thin gate electrode and method of fabricating same

ABSTRACT

A field effect transistor and a method of fabricating the field effect transistor. The field effect transistor includes: a silicon body, a perimeter of the silicon body abutting a dielectric isolation; a source and a drain formed in the body and on opposite sides of a channel formed in the body; and a gate dielectric layer between the body and an electrically conductive gate electrode, a bottom surface of the gate dielectric layer in direct physical contact with a top surface of the body and a bottom surface the gate electrode in direct physical contact with a top surface of the gate dielectric layer, the gate electrode having a first region having a first thickness and a second region having a second thickness, the first region extending along the top surface of the gate dielectric layer over the channel region, the second thickness greater than the first thickness.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor devices;more specifically, it relates to a field effect transistor having a thingate electrode and a method for fabricating the field effect transistor.

BACKGROUND OF THE INVENTION

As the field effect transistors (FETs) used in integrated circuitsbecome ever smaller, it has been found that many parameters do notdecrease (or scale) as the physical dimensions of the FET decrease. Oneof these parameters is the fringe capacitance between the source/drainsof the FET and the gate electrode. As capacitance increases, FETs slowdown. Since fringe capacitance does not scale, smaller FETs do notexhibit as much increase in speed as expected. Thus, to achievecontinuing performance gain with decreasing FET dimensions there is aneed for FET structures having reduced fringe capacitance.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a field effect transistor,comprising: a silicon body, a perimeter of the silicon body abutting adielectric isolation; a source and a drain formed in the body and onopposite sides of a channel formed in the body; and a gate dielectriclayer between the body and an electrically conductive gate electrode, abottom surface of the gate dielectric layer in direct physical contactwith a top surface of the body and a bottom surface the gate electrodein direct physical contact with a top surface of the gate dielectriclayer, the gate electrode having a first region having a first thicknessand a second region having a second thickness, the first regionextending along the top surface of the gate dielectric layer over thechannel region, the second thickness greater than the first thickness.

A second aspect of the present invention is a method of fabricating afield effect transistor, comprising: forming a dielectric isolationalong a perimeter of a region of a silicon layer to define a siliconbody in the silicon layer; forming a gate dielectric layer in directphysical contact with a top surface of the silicon body; forming a gatedielectric layer on the silicon body, a bottom surface of the gatedielectric layer in direct physical contact with a top surface of thesilicon body; and forming an electrically conductive gate electrode onthe gate dielectric layer, bottom surface of the gate electrode indirect physical contact with a top surface of the gate dielectric layer,the gate electrode having a first region having a first thickness and asecond region having a second thickness, the first region extendingalong the top surface of the gate dielectric layer over the channelregion, the second thickness greater than the first thickness.

BRIEF DESCRIPTION OF DRAWINGS

The features of the invention are set forth in the appended claims. Theinvention itself, however, will be best understood by reference to thefollowing detailed description of an illustrative embodiment when readin conjunction with the accompanying drawings, wherein:

FIGS. 1 through 7 are cross-sectional drawings illustrating fabricationof an FET according to a first embodiment of the present invention.

FIG. 8A is a cross-section through line 8A-8A of FIG. 8B which is a topview of the FET of FIGS. 1 through 7 after a further fabrication step;

FIG. 9A is a top view and FIG. 9B is a cross-section through line 9B-9Bof FIG. 9A of first alternative layout of an FET according to the firstembodiment of the present invention;

FIG. 10A is a top view and FIG. 10B is a cross-section through line10B-10B of FIG. 10A of second alternative layout of an FET according tothe first embodiment of the present invention;

FIGS. 11 through 13 are cross-sectional drawings illustratingfabrication of an FET according to a second embodiment of the presentinvention; and

FIG. 14A is a cross-section through line 14A-14A of FIG. 14B, which is atop view of the FET of FIGS. 11 through 13 after a further fabricationstep.

DETAILED DESCRIPTION OF THE INVENTION

All etch steps described infra, unless otherwise noted, may be performedusing a reactive ion retch (RIE) process.

FIGS. 1 through 7 are cross-sectional drawings illustrating fabricationof an FET according to a first embodiment of the present invention. InFIG. 1, a silicon-on-insulator (SOI) substrate includes a lower siliconlayer 105, a single-crystal upper silicon layer 110 and a buried oxidelayer (BOX) 115 between the upper and lower silicon layers. Formed inupper silicon layer 110 is a shallow trench isolation 120 surrounding asingle-crystal silicon body region 125. Formed on a top surface SOIsubstrate 100 is a gate dielectric layer 130. Formed on a top surface ofgate dielectric layer 130 is a polysilicon layer 135.

Polysilicon layer 135 has a thickness of T1. In one example T1 isbetween about 20 nm and about 100 nm. In one example gate dielectriclayer 130 is silicon dioxide (Oxidized silicon), silicon nitride(Si₃N₄), silicon oxynitride (SiO_(Y)N_(X)) or combinations of layersthereof. In one example, gate dielectric layer 130 is a high K(dielectric constant) material, examples of which include, but are notlimited to, metal oxides such as Ta₂O₅, BaTiO₃, HfO₂, ZrO₂, Al₂O₃, ormetal silicates such as HfSi_(x)O_(y) or HfSi_(x)O_(y)N_(z) orcombinations of layers thereof. A high K dielectric material has arelative permittivity above about 10. In one example, gate dielectriclayer 130 is about 0.5 nm to 20 nm thick.

In FIG. 2, a hardmask layer 140 is formed on top surface 145 ofpolysilicon layer 135 and an opening 150 formed in the hardmask layerexposing the top surface of the polysilicon layer. In one example,hardmask layer 140 is Si₃N₄.

In FIG. 3, an oxidation of silicon layer 135 is performed to form aOxidized silicon region 155 in opening 150. The oxidation is controlledso as not consume all the polysilicon in opening 150, but leave a thinpolysilicon layer 160 over gate dielectric layer 130. In one example T2is less than or equal to about 50 nm. In one example, T2 is less than orequal to about 30 nm.

In FIG. 4, polysilicon layer 135 (see FIG. 3) and dielectric layer 140are masked (using conventional photolithography to form a patternedphotoresist mask) and then polysilicon layer 135 and dielectric layer140, but not gate dielectric layer 130, are etched where not protectedby the photoresist mask to define a first gate electrode precursorstructure 166 comprising a thick polysilicon region 165, thinpolysilicon layer 160 and Oxidized silicon region 155. Gate electrodeprecursor structure 166 that will define the lateral extents of the gateelectrode of the FET being fabricated. A lateral direction is anydirection parallel to top surface 150. Thin polysilicon layer 160completely overlaps STI 120 over a first pair of opposite sidewalls(sidewalls 167A and 167B) of body 125 but does not completely overlapSTI 120 over a second pair of opposite sidewalls of body 125 (not shownin FIG. 4). The first and second pairs of sidewalls are mutuallyperpendicular. Between each sidewall of the second pair of sidewalls andcorresponding opposite sides of thin polysilicon layer 160 exist regionsof body 125 not covered by thin polysilicon layer 160 and Oxidizedsilicon region 155. It is in these regions of body 125 that thesource/drains of the FET being fabricated will be formed.

After the masking and etching, the photoresist mask is removed andspacers 170 are formed on the sidewalls of first gate electrodeprecursor structure 166 and source/drain ion implantations performedinto regions of body 125 not protected by spacers 170, or first gateelectrode precursor structure 166. Spacers 170 may be formed bydeposition of a conformal material flowed by a directional RIE process.Spacers 170 consist of a dielectric material. Spacers 170 may comprisemultiple independently formed spacers and multiple source/drain ionimplantation steps may be performed, including source/drain extensionimplants and halo implants as commonly known in the art.

Next, regions of gate dielectric layer 130 not protected by spacers 170or first gate electrode precursor structure 166 are removed and a metalsilicide layer formed on the source drains. In one example, the silicidelayer comprises Pt, Ti, Co or Ni silicide. Metal silicides may beformed, by blanket deposition of a thin metal layer followed by heatingto a temperature sufficient to cause a chemical reaction between themetal and any silicon layer in contact with the metal, followed by RIEor wet etching to remove any unreacted metal.

In FIG. 5, a dielectric layer 175 is formed over spacers 175, first gateelectrode precursor structure 166 and exposed regions of substrate 100and STI 120. Then a chemical-mechanical-polish (CMP) is performed toexpose first gate electrode precursor structure 166 using hardmask layer140 (see FIG. 4) as a polish stop. Next any remaining hardmask layer 140is removed, by RIE or wet etching. In one example dielectric layer 175comprises silicon dioxide (Oxidized silicon), silicon nitride (Si₃N₄),silicon carbide (SiC), silicon oxy nitride (SiON), silicon oxy carbide(SiOC), organosilicate glass (SiCOH), plasma-enhanced silicon nitride(PSiN_(x)) or NBLok (SiC(N,H)) or combinations of layers thereof.

In FIG. 6, Oxidized silicon region 155 (see FIG. 5) is removed by RIE orwet etching to form a second gate electrode precursor structure 176comprising thick polysilicon region 165 and thin polysilicon layer 160.

In FIG. 7, a gate electrode 177 is formed. Gate electrode 177 comprisesa continuous metal silicide layer 180 and thick polysilicon region 165.Thin polysilicon layer 160 (see FIG. 6) has been totally consumed by themetal silicide formation process. A thin gate electrode region 178 ofgate electrode 177 comprises a first region of metal silicide layer 180in direct physical contact with gate dielectric layer 130 over body 125.A raised gate electrode region 179 of gate electrode 177 comprises asecond region of metal silicide layer 180 in direct contact with thickpolysilicon region 165. In one example, silicide layer 180 comprises Pt,Ti, Co or Ni silicide. Raised contact region 179 provides a surfacehigher than thin gate electrode region 178 (relative to the top surfaceof gate dielectric layer 130) in order to land a gate contact asillustrated in FIG. 8A and described infra. This prevents breakthroughof the gate contact into gate dielectric layer 130.

Silicide layer 180 and thus thin gate electrode region 178 of gateelectrode 177 has a thickness of T3 and raised gate electrode region 179of gate electrode 177 has a thickness T4. In one example T3 is less thanor equal to about 40 nm. In one example, T3 is less than or equal toabout 20 nm. T4 is always greater than T3. In one example T4 is greaterthan or equal to about twice T3. Except for gate and source draincontacts; fabrication of an FET 182 is essentially complete.

FIG. 8A is a cross-section through line 8A-8A of FIG. 8B, which is a topview of the FET of FIGS. 1 through 7 after a further fabrication step.In FIG. 8A, a dielectric layer 185 is formed over dielectric layer 175and FET 182 and a CMP performed to planarize dielectric layer 175. Inone example gate dielectric layer 175 comprises Oxidized silicon, Si₃N₄,SiO_(Y)N_(X), a low K (dielectric constant) material, examples of whichinclude but are not limited to hydrogen silsesquioxane polymer (HSQ),methyl silsesquioxane polymer (MSQ), SiLK™ (polyphenylene oligomer)manufactured by Dow Chemical, Midland, Tex., Black Diamond™ (methyldoped silica or SiO_(x)(CH₃)_(y) or SiC_(x)O_(y)H_(y) or SiOCH)manufactured by Applied Materials, Santa Clara, Calif., organosilicateglass (SiCOH), and porous SiCOH and combinations of layers thereof. Alow K dielectric material has a relative permittivity of about 2.4 orless. In one example, dielectric layer 175 is between about 300 nm andabout 2,000 nm thick.

Next an electrically conductive gate contact 190A is formed from a topsurface of dielectric layer down to at least metal silicide layer 180over thick polysilicon region 165, for example, by a damascene process.In one example, contact 190A comprises W, Ta, tantalum nitride (TaN),Ti, titanium nitride (TiN) or combinations of layers thereof. Gatecontact 190A may extend into or through metal silicide layer 180 intothick polysilicon layer 165. Advantageously, there is no gate contact tothin gate electrode region 178, only a gate contact to raised gateelectrode region 179. In FIG. 8B, dielectric layers 175 and 185 areomitted for clarity. In FIG. 8B, source/drains 195A and 195B are formedon either side of gate electrode 177 and contacts 190A and 190B (similarto contact 190A) are formed to source/drains 195A and 195B. FET 182 thusfabricated has a gate channel length L_(G) defined by the width of gateelectrode 177 between source/drains 195A and 195B. There is also aphysical channel length L_(PHYSICAL) that is defined as the distancebetween source/drains 195A and 195B. L_(G) is greater than or equal toL_(PHYSICAL) and depends upon how far source/drains 195A and 195B extendunder gate electrode 177. In one example, (referring back to FIG. 7) T3divided by L_(G) is less than or equal to 1. In one example, L_(G) isequal to or greater than about 4 times T3. In one example, (referringback to FIG. 7) T3 divided by L_(PHYSICAL) s less than or equal to 1. Inone example, L_(PHYSICAL) is equal to or greater than about 4 times T3.

In FIGS. 8A and 8B, raised gate electrode region 179 is completelyformed over STI 120 and no portion of raised gate electrode region 179overlaps body 125. Only thin gate electrode region 178 extends over body125. The fringe capacitance is thus reduced because of thinness of thingate electrode region 178 over body 125 of FET 182 compared toconventional FETs where a thick polysilicon layer would extend out overthe silicon body of the conventional FET.

FIG. 9A is a top view and FIG. 9B is a cross-section through line 9B-9Bof FIG. 9A of first alternative layout of an FET according to the firstembodiment of the present invention. FIG. 9A is similar to FIG. 8B andFIG. 9B is similar to FIG. 8A except that a region of raised gateelectrode region 179 of an FET 182A overlaps STI 120 and body 125. Thisstill results in reduced fringe capacitance compared to a conventionalFET and allows a reduction is area of FET 182A compared to FET 182 ofFIGS. 8A and 8B.

FIG. 10A is a top view and FIG. 10B is a cross-section through line10B-10B of FIG. 11A of second alternative layout of an FET according tothe first embodiment of the present invention. FIG. 10A is similar toFIG. 8B and FIG. 10B is similar to FIG. 8A except that raised gateelectrode region 179 of an FET 182B is formed completely over body 125.This still results in reduced fringe capacitance compared to aconventional FET and allows a reduction is area of FET 182B compared toFET 182 of FIGS. 8A and 8B and FET 182A of FIGS. 9A and 9B.

FIGS. 11 through 13 are cross-sectional drawings illustratingfabrication of an FET according to a second embodiment of the presentinvention. FIG. 11 is similar to FIG. 1 except polysilicon layer 135 ofFIG. 1 is replaced with a metal layer 200 having a thickness T5. In oneexample, metal layer 200 comprises Al, Ti, W, Ta, TiN, or TaN.

In FIG. 12, a mask 205 is formed on top surface 215 of metal layer 200and a trench 215 partially etched into the metal layer. A thus thinnedregion 220 of metal layer 200 has a thickness T6. Mask 205 may be apatterned photoresist layer formed by conventional photolithographictechniques well know in the art or may be a patterned hardmask layerformed by conventional photolithographic and etching techniques wellknow in the art.

Referring to FIGS. 11 and 12, in one example T5 is less than or equal toabout 40 nm. In one example, T5 is less than or equal to about 20 nm. T5is always greater than T6. In one example T5 is greater than or equal toabout twice T6.

In FIG. 13, mask 205 of FIG. 11 is removed, a blanket dielectric layerdeposited over metal layer 200, the blanket dielectric layer is masked(using conventional photolithography to form a patterned photoresistmask) and then the blanket dielectric layer and gate dielectric layer130 are etched, where not protected by the photoresist mask, to define agate electrode precursor structure 225 comprising a thick metal region230, a thin metal region 235 and a dielectric mask 240. Gate electrodeprecursor structure 225 will define the lateral extents of the gateelectrode of the FET being fabricated. Thin metal region 235 completelyoverlaps STI 120 over a first pair of opposite sidewalls (sidewalls 167Aand 167B) of body 125 but does not completely overlap STI 120 over asecond pair of opposite sidewalls of body 125 (not shown in FIG. 13).The first and second pairs of sidewalls are mutually perpendicular.Between each sidewall of the second pair of sidewalls and correspondingopposite sides of thin metal region 235 exist regions of body 125 arenot covered by thin metal region 235 and dielectric layer 240. It is inthese regions the source/drains of the FET being fabricated will beformed.

Next, source/drains are formed and metal silicide layers formed on thesource/drains as described supra. Formation of sidewall spacers isoptional.

FIG. 14A is a cross-section through line 14A-14A of FIG. 14B, which is atop view of the FET of FIGS. 11 through 13 after a further fabricationstep. In FIG. 14B, dielectric layers 175 and 185 are omitted forclarity. In FIGS. 14A and 14B dielectric layer 240 (see FIG. 13) hasbeen removed to form a gate electrode 245 comprising thick metal region230 and thin metal region 235. Dielectric layers 175 and 185 have beenformed as well as contacts 190A, 190B and 190C to form a FET 250.

FET 250 thus fabricated has a gate channel length L_(GM) defined by thewidth of gate electrode 245 between source/drains 195A and 195B. Thereis also a physical channel length L_(PHYSICAL) that has been describedsupra. L_(GM) is greater than or equal to L_(PHYSICAL) and depends uponhow far source/drains 195A and 195B extend under gate electrode 245. Inone example, (referring back to FIGS. 11 and 12) T5 divided by L_(GM) isless than or equal to 1. In one example, L_(GM) is equal to or greaterthan about 4 times T5. In one example, (referring back to FIGS. 11 and12) T5 divided by L_(PHYSICAL) is less than or equal to 1. In oneexample, L_(PHYSICAL) is equal to or greater than about 4 times T5.

The alternatives layouts illustrated in FIGS. 9A, 9B, 10A and 10B forthe first embodiment of the present invention are equally applicable tothe second embodiment of the present invention.

Thus, the embodiments of the present invention provide FET structureshaving reduced fringe capacitance.

The description of the embodiments of the present invention is givenabove for the understanding of the present invention. It will beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of various modifications,rearrangements and substitutions as will now become apparent to thoseskilled in the art without departing from the scope of the invention.Therefore, it is intended that the following claims cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

1. A method of fabricating a field effect transistor, comprising:forming a dielectric isolation along a perimeter of a region of asilicon layer to define a silicon body in said silicon layer; forming agate dielectric layer on said silicon body, a bottom surface of saidgate dielectric layer in direct physical contact with a top surface ofsaid silicon body; and forming an electrically conductive gate electrodeon said gate dielectric layer, bottom surface of said gate electrode indirect physical contact with a top surface of said gate dielectriclayer, said gate electrode having a first region having a firstthickness and a second region having a second thickness, said firstregion extending along said top surface of said gate dielectric layerover said channel region, said second thickness greater than said firstthickness; and wherein said forming said gate electrode includes:forming a polysilicon layer on said top surface of said gate dielectriclayer; oxidizing a less than whole thickness of said polysilicon layerin a region of said polysilicon layer over said body to form an oxidizedpolysilicon layer; after forming said source and said drain, removingsaid oxidized polysilicon layer to form said first region of said gateelectrode; and simultaneously converting said first region of said gateelectrode to a metal silicide layer and forming said metal silicidelayer on exposed surfaces of said second region of said gate electrode.2. The method of claim 1, wherein a top surface of said second region ofsaid gate electrode is further away from said top surface of said gatedielectric layer than a top surface of said first region of said gateelectrode is away from said top surface of said gate dielectric layer.3. The method of claim 1, wherein said second region does not overlapsaid silicon body.
 4. The method of claim 1, wherein said second regionoverlaps at least a region of said silicon body.
 5. The method of claim1, wherein said first thickness is less than or equal to about 50 nm. 6.The method of claim 1, wherein a channel length of said field effecttransistor is defined by a width of said gate electrode between saidsource and said drain, and wherein said first thickness divided by saidchannel length is less than or equal to one.
 7. The method of claim 1,wherein a channel length of said field effect transistor is defined by awidth of said gate electrode between said source and said drain, andwherein said channel length is greater than or equal to four times saidfirst thickness.
 8. The method of claim 1, further including: forming agate electrode contact in direct physical and electrical contact onlywith said second region of said gate electrode.
 9. A method offabricating a field effect transistor, comprising: forming a dielectricisolation along a perimeter of a region of a silicon layer to define asilicon body in said silicon layer; forming a gate dielectric layer onsaid silicon body, a bottom surface of said gate dielectric layer indirect physical contact with a top surface of said silicon body; andforming an electrically conductive gate electrode on said gatedielectric layer, bottom surface of said gate electrode in directphysical contact with a top surface of said gate dielectric layer, saidgate electrode having a first region having a first thickness and asecond region having a second thickness, said first region extendingalong said top surface of said gate dielectric layer over said channelregion, said second thickness greater than said first thickness; andwherein said forming said gate electrode includes: forming a metal layeron said top surface of said gate dielectric layer; etching away a lessthan whole thickness of said metal layer in a region of said metal layerover said body to form said first region of said gate electrode; forminga dielectric layer over said first region; and after forming said sourceand said drain, removing said dielectric layer.
 10. The method of claim9, wherein a top surface of said second region of said gate electrode isfurther away from said top surface of said gate dielectric layer than atop surface of said first region of said gate electrode is away fromsaid top surface of said gate dielectric layer.
 11. The method of claim9, wherein said second region does not overlap said silicon body. 12.The method of claim 9, wherein said second region overlaps at least aregion of said silicon body.
 13. The method of claim 9, wherein saidfirst thickness is less than or equal to about 50 nm.
 14. The method ofclaim 9, wherein a channel length of said field effect transistor isdefined by a width of said gate electrode between said source and saiddrain, and wherein said first thickness divided by said channel lengthis less than or equal to one.
 15. The method of claim 9, wherein achannel length of said field effect transistor is defined by a width ofsaid gate electrode between said source and said drain, and wherein saidchannel length is greater than or equal to four times said firstthickness.
 16. The method of claim 9, further including: forming a gateelectrode contact in direct physical and electrical contact only withsaid second region of said gate electrode.